Effects Of Dissolved Organic Matter On Phosphate Adsorption By Lanthanum Modified Bentonite

With the development of society and the needs of human activities,more and more phosphorus-containing substances will discharge into water inevitably and easily lead to eutrophication.Lanthanum modified bentonite（LMB）is widely used to adsorb inorganic phosphorus in polluted water.However,the adsorption performance of LMB is affected by dissolved organic matter（DOM）.In this thesis,humic acid（HA）which was the key influencing factor was selected out by studying the influence of the type and proportion of DOM on phosphate adsorption by LMB.Then,the kinetics and isotherms of HA and phosphate adsorption on LMB were carried out.The adsorption behavior and mechanism of the two adsorbates on LMB were analyzed by using SEM-EDS,XRD,FTIR and XPS to characterize LMB.Finally,the adsorption behavior of HA and phosphorus on LMB in different ways of adding adsorbents,and the long-term effects of LMB on DOM and phosphate in natural water were studied.The main conclusions obtained are as follows:（1）In the condition that phosphate and one type of DOM among HA,bovine serum albumin（BSA）and sodium alginate（SA）coexisted,only HA exerted an effect on the phosphate adsorption by LMB,which was an inhibition;simultaneously,LMB adsorbed HA regularly.When three DOM were mixed with phosphate in different proportions and BSA or SA was the main DOM,it had nearly no effect on the phosphate removal by LMB.When DOM was mainly HA,the performance of phosphate adsorption on LMB became worse.（2）The kinetics of HA or phosphate adsorption on LMB were corresponded with the pseudo-second-order kinetic model,and isotherms matched Langmuir model;the maximum adsorption capacity of phosphate was 5.70 mg g^-1,and HA was 11.83 mg g^-1.In the mixed solution of HA and phosphate with a C/P molar ratio of 5.35,isotherms of HA and phosphate adsorption on LMB still conformed to Langmuir model,but the maximum adsorption capacity of HA was only 4.51 mg g^-1,LMB preferentially adsorbed phosphate.（3）XPS analysis found that after adsorbing BSA,LMB may exist protonated amino functional groups that electrostatically attracted to phosphate.Both HA and SA contain functional groups such as C-O/C-OH,but HA with a smaller molecular size may diffuse into the adsorbent,resulting in hindering phosphate adsorption.La and HA formed La-HA complexes.O-C=O bonds of HA competed for lanthanum with phosphate,but LMB had a better binding ability with phosphate when co-adsorption began at C/P molar ratio of 5.35,namely adsorbate added according to maximum adsorption amount.It may form La-P-HA ternary complexes during co-adsorption to keep HA adsorption amount.La PO₄·0.5H₂O appeared in the XRD pattern of this experimental group.Moreover,O-P-O bond appeared after phosphate adsorption,forming La-O-P inner-sphere complexes was the main mechanism of phosphate removal by LMB.（4）Whether phosphate added first and then HA,or adding them together,the presence of HA was disadvantage to phosphate removal by LMB.When adding adsorbates by two steps,La in LMB may release during adding 3-10 mg L^-1 HA;but the adsorption capacity of phosphate increased with HA concentration rising,perhaps resulting from the formation of hydrogen bonding between them.The adsorption capacity of HA was higher during co-adsorption,and its isotherm conformed to Freundlich model.Comparing co-adsorption with adding HA first and then phosphate,the HA adsorption capacity under co-adsorption condition was lower.Setting the phosphate concentration of 1 mg L^-1 as the dividing line,the adsorption amount of HA was lower when the phosphate concentration was higher than this,while the adsorption capacity of phosphate was higher.It was related to the excess La at low phosphate concentration.Phosphate can always bind to LMB stably,and when it was higher than 1 mg L^-1,the phosphate robbing the adsorption sites can provide the adsorption locations for HA.（5）Researching excessive pollution river water with humus as the fluorescent DOM,phosphate concentration decreased with the extension of reaction time;the higher initial concentration of DOM,the slower phosphate concentration decreased.At the same time,the fluorescence intensity of DOM increased with the progress of reaction.LMB can adsorb humic acid in the natural water.